1. Field of the Invention
This invention relates to the processing of mixed hydrocarbon gasoline feedstocks to improve octane ratings. More specifically, the process uses integrated steps of adsorption, isomerization, and saturation to catalytically isomerize normal paraffin hydrocarbons, to concentrate non-normal hydrocarbons in a product stream, and to saturate aromatic hydrocarbons to meet benzene product limits.
2. Description of the Prior Art
Recent concerns about volatility and toxicity of hydrocarbon fuel and the resultant environment damage has prompted legislation that will limit the content and composition of aromatic hydrocarbons in such fuels. Many of these limitations relate specifically to benzene which, due to its toxicity, will be substantially eliminated from the gasoline pool. These new fuels are generally referred to as reformulated gasolines. New requirements for reformulated gasoline have profound impacts on the operation of refinery processes for producing high octane fuels. Reformulated gasoline requirements impose limitations on gasoline end points, benzene as well as total aromatics, and reid vapor pressure (RVP).
Benzene reduction will disrupt the current operation methods of several hydrocarbon processes that produce high octane fuel. Benzene reduction poses some of the most severe problems for the operation of catalytic reformers that have been relied on to raise the octane of unleaded fuels. Reformers that were typically operated at high severity to produce high octane benzene and aromatic hydrocarbons must now be operated to eliminate benzene and reduce aromatics. Reforming converts the C.sub.6 and heavier hydrocarbons to aromatic compounds. Octane improvement is also obtained by catalytically isomerizing the paraffinic hydrocarbons to rearrange the structure of the paraffinic hydrocarbons into branch-chained paraffins such as normal C.sub.5 hydrocarbons which are not readily converted into aromatics. Although the non-cyclic C.sub.6 and heavier hydrocarbons can be upgraded into aromatics through dehydrocyclization, the conversion of C.sub.6 's to aromatics creates higher density species and increases gas yields with both effects leading to a reduction in liquid volume yields. Therefore, it is preferable to charge the non-cyclic C.sub.6 paraffins to an isomerization unit to obtain C.sub.6 isoparaffins hydrocarbons. Consequently, octane upgrading commonly uses isomerization to convert normal C.sub.6 and lighter boiling hydrocarbons and reforming to convert C.sub.6 cycloparaffins and higher boiling hydrocarbons. However, both reforming and isomerization processes streams often require treatments for benzene elimination.
Elimination of benzene can be accomplished by removing benzene or benzene precursors from hydrocarbons prior to hydrocarbon conversion processes or removing benzene from the product stream by saturation, separation or conversion. Methods for eliminating benzene from a reforming effluent include direct saturation or saturation through isomerization. It is well known to eliminate benzene by direct saturation of product or feed streams such as an isomerization zone feed. U.S. Pat. No. 5,003,118 teaches a process for the directly saturating benzene in a benzene saturation reactor and passing the remainder of the stream as feed to an isomerization zone.
The benzene contribution from the reformate portion of the gasoline pool can also be decreased or eliminated by altering the operation of the reforming section. There are a variety of ways in which the operation of the reforming section may be altered to reduce the reformate benzene concentration. Changing the cut point of the naphtha feed split between the reforming and isomerization zones from 180.degree. to 200.degree. F. will remove benzene, cyclohexane and methylcyclopentane from the reformer feed. Benzene can alternately also be removed from the reformate product by splitting the reformate into a heavy fraction and a light fraction that contains the majority of the benzene. Practicing either method will put a large quantity of benzene into the feed to other processing units, particularly the isomerization zone. Therefore, it is necessary to have an efficient and cost effective means for removing benzene from feeds.
The saturation of benzene for an isomerization process imposes several costs and process drawbacks. Where a saturation effluent from a benzene saturation reactor passes directly to isomerization reactors, the effluent must be brought to the isomerization reactor pressure. In addition for water sensitive catalysts, the hydrogen used in the benzene saturation must also undergo drying to keep water out of the feed to the isomerization reactors. When the concentration of benzene that requires isomerization increases, additional drying and compressor cost become significant due to the high relative hydrogen requirements to satisfy the stoichiometric requirements for benzene saturation. In addition, the heat integration important to isomerization reactors does not permit full recovery of the heat of reaction released in the benzene saturation reactor. In fact, high benzene concentrations in the isomerization feed result in excessive temperatures that interfere with the isomerization process by producing coking, poor selectivity, and short catalyst cycles. Therefore, it is desirable to eliminate benzene from the feed to the isomerization zone.
Technology for isomerizing hydrocarbons is well known and often uses a series of adsorbers to remove non-normals in the production of a normal hydrocarbon-rich recycle stream that returns to the isomerization zone. Operations of this type are well known and described in U.S. Pat. No. 4,929,799, the contents of which are hereby incorporated by reference.
It is an object of this invention to provide an integrated process for the saturation of benzene, isomerization of light paraffins, and adsorptive enrichment of a normal hydrocarbon recycle stream, that operates with greater efficiency and effectiveness than over those of the prior art.
It is a further object of this invention to provide a process for raising the octane of gasoline range naphtha streams by isomerization, adsorption, and benzene saturation without raising temperatures in the isomerization reaction zone.